Applicants claim priority under 35 USC 119 of German Application No. 199 10 582.0, filed on Mar. 10, 1999. Applicants also claim priority under 35 USC 165 of PCT/DE00/00199, filed on Jan. 25, 2000. The international application under PCT article 21(2) was not published in English.
1. Field of the Invention
The invention concerns multipiece pistons.
2. The Prior Art
A piston of this type can be inferred from the abstract of DE-A-44 16 120 as a possible implementation. Lower parts made of steel have, however, been viewed by those skilled in the art as impractical for reasons of production up until now, particularly for pistons with larger diameters.
Multipiece pistons know from practice, in which a lower part is screwed onto an upper part in a method according to this type, typically have lower parts made of pressed aluminum or cast iron. It has been shown that the known lower parts are increasingly reaching the limits of their strength and that, particularly for lower parts make fo cast iron, these limits are caused by casting defects which, in spite of increased expenditures for testing, can hardly be prevented.
The invention therefore concerns itself with the problem of preparing a lower part for pistons of this type which meets the increased requirements for strength while simultaneously being economically producible.
Through the production of the lower part using forging technology in connection with the material precipitation-hardened, ferrite-pearlite steel, an economically producible lower part with high strength is provided.
Because the microstructure of the precipitation-hardened ferrite-pearlite steel is achieved through appropriate heat control during the forging process, in larger components with corresponding wall thicknessesxe2x80x94the lower parts of multipiece pistons of greater than 250 mm in diameter are included in thesexe2x80x94the danger arises that the desired microstructure will not be achieved over the entire cross section, and the strength will turn out to be lower than expected.
Titanium can be added to the precipitation-hardened, ferrite-pearlite steel as a means to achieve the desired microstructurexe2x80x94particularly in regard to the grain size. The steel 38 MnVs6 has been shown to be an advantageous precipitation-hardened ferrite-pearlite steel.
The use of bushings in the region of the hubs can be dispensed with if the piston pins and/or the pin hubs are coated. Phosphatizing is particularly considered in this regard.
In addition, through the material of the lower part according to the invention, it is also possible to increase the surface pressures n the region of the pins.
The lower length of the multipiece piston is advantageously reduced to less than 40% of the piston diameter, because in this way weight is saved and forging technology problems are avoided. Nonetheless, due to the higher module of elasticity of steel compared to gray cast iron and aluminum, a uniform straight-line motion is achieved.
One or more recesses to receive the cooling oil are introduced into the bottom of the lower part by machining.
The diameter range for lower parts according to the invention begins at approximately 160 mm. Conditioned by the icreased material strength, the wall thickness canxe2x80x94compared with cast lower partsxe2x80x94be lower locally. In any case, higher wall thicknesses are also achieved in some parts through the taper bevels necessary for forging.